3/4 leaves 1/4 of the log left, you then have to divide 1/4 in half, so that would be 0.5/4 (1/8=0.5/4), so you add the amount cut off together (3/4+0.5/4= 3.5 fourths) to get 0.5/4 which is equal to 1/8 of the log.
Also, you could make the 3/4 easier to do math with by using 6/8 (3/4=6/8 btw I multiplied the denominator and the numerator by 2) instead. That then leaves the 1/8 of the log left. (Equation forms: 6/8+1/8=7/8 8/8-7/8=1/8)
Final answer: 1/8 (or 0.125) of the original log remains.
How to find decimal form: divide the denominator by the numerator, so in this case, 8 divided by 1=0.125
Hope this helped, let me know if I need to explain anything else